Many aircraft are designed to fly at high altitudes, e.g., from 10,000 feet (ft) (about 3,000 meters (m)) to upwards of 41,000 ft (about 12,500 m), while providing a safe, comfortable cabin environment. To maintain this cabin environment, a typical aircraft includes an environmental control system, with appropriate pressure, temperature, and moisture regulation, to circulate fresh air within the cabin (e.g., the flight deck, also called the cockpit, the passenger cabin, and/or the cargo areas). An environmental control system may be referred to as an air handling system, an air circulation system, and/or an air conditioning system. An environmental control system may circulate outside air mixed with an amount of highly filtered air from the cabin. The combined outside and filtered air is ducted to the cabin and distributed to cabin outlets throughout the cabin. Air diffusers direct the ducted air into the cabin outlets.
Inside the cabin, cabin outlets are generally arranged along the side walls of the cabin and sometimes along the overhead. The air flows in generally circular patterns and exits through cabin outlets, often on either side of the cabin floor, and, on airplanes with overhead recirculation, the air may exit through overhead cabin outlets. For commercial aircraft, the FAA (Federal Aviation Administration) requires a minimum air flow and cabin pressure. For new aircraft, the minimum air flow is 0.55 pounds per minute (lbs/min) per occupant (about 250 grams per minute (g/min) per occupant) and the minimum cabin pressure is 0.75 bar (75 kilopascal (kPa)).
The cabin air flow is continuous and is used for maintaining a comfortable cabin temperature, pressurization, and/or overall air quality. Some portion of the air exiting the cabin is exhausted from the airplane through one or more outflow valves in the fuselage, which also controls the cabin pressure. The remaining portion is drawn through high efficiency filters, and then is recirculated with fresh outside air, as discussed.
For passenger comfort, air flow into the cabin should be quiet, relatively uniform, and generally unobtrusive. However, in addressing these needs, designers must balance weight and complexity with comfort.